A buffer layer such as CdS is advantageously utilized to produce a reproducible and efficient heterojunction in thin film solar cells. CdS buffer layers also find application in other types of solar cells. Solar cells are photovoltaic components for direct generation of electrical current from sunlight. Due to the growing demand for clean sources of energy, the manufacture of solar cells has expanded dramatically in recent years. A variety of solar energy collecting modules currently exists. One such module includes a photovoltaic panel that receives solar energy and converts the solar energy directly into electricity. Another such module includes a solar thermal collecting panel which harnesses solar energy for heat. The solar energy collecting modules can have different geometries and be formed of different materials, but generally consist of large, flat solar panels and include an absorber layer.
CdS layers are advantageously utilized as buffer layers in solar cells. CIGS-type solar cells utilize a CdS buffer layer disposed between a ZnO window layer and a CIGS (Cu(In,Ga)Se2) absorber layer. Integrating a CdS layer into the ZnO/CIGS system enhances the spectral absorption of the solar cell, since the refractive index of CdS (nr≈2.4) lies between the refractive indices of ZnO (nr≈1.9) and CIGS (nr≈2.9). The large step between the refractive indices of ZnO and CIGS is therefore divided into two smaller steps resulting in an overall reduced reflectivity of the solar cell. Because of the role that the CdS layer plays, it is important to form the CdS layer to have desired qualities and thicknesses. As a corollary, it is important to analyze and accurately measure or monitor the thickness of the CdS buffer layers. The same is true for other buffer layers in other thin film solar cell embodiments.
Current methods for monitoring the thickness of buffer layers such as CdS, in solar cells include SEM (Scanning Electron Microscope) and TEM (Transmission Electron Microscope) evaluation of thickness. Each of these techniques is destructive, expensive and time consuming. Many other systems that measure EQE (External quantum efficiency) are also expensive and involve time consuming processes and dedicated equipment.
It would be desirable to provide accurate methods and systems for evaluating CdS and other film thicknesses in a non-destructive manner.